It is desirable to understand electrostatic phenomena observed with moving dielectric and slightly conductive sheets and webs passing over conveyance rollers. For example, it is desirable, in connection with certain coating technologies, especially film manufacturing, to know how much surface charge is on a sheet or web surface just prior to coating.
An apparatus and method for measuring net charge distribution in the lengthwise direction of the web, that is, in the direction of conveyance of the web over the roller, is described in the article entitled "Segmented Roller: A Device for Measuring Charge Density on a Moving Conducting Web" published in the Journal of Electrostatics, Vol. 24, pages 21-32 (1989). Measurement of the charge distribution in the lateral direction of the web is described by the article entitled "The Split Segmented Roller: A Device for Measuring Lateral Charge Distribution on a Moving Web" published in the Journal of Electrostatics, Vol. 28, pages 39-45 (1992) and is the subject of U.S. Pat. No. 5,066,918. While both devices described can measure net charge on a moving web, no capability exists for measuring the polar or bound charge density on the web.
The present invention provides an apparatus capable of measuring surface charge density which is used to determine the polar charge density on a non-moving sheet, and an apparatus with the same capability for a moving sheet or web.